This invention relates to thermoplastic formulations containing a blend of polyphenylene ether and a vinyl aromatic polymer which are flame retardant and have improved melt flow.
Thermoplastic formulations based upon a blend of a polyphenylene ether and a polyvinyl aromatic are well known. Such formulations are useful in engineering thermoplastic applications.
The polyphenylene ether constituent of the blend is frequently poly(2,6-dimethyl-1,4-phenylene ether) or the 2,5-dimethyl isomer thereof. The polyphenylene ether may also be a copolymer, such as the copolymer produced from 2,6-dimethylphenol and 2,3,6-trimethylphenol. These and other suitable polyphenylene ethers and their preparations are discussed, for example, in U.S. Pat. Nos. 3,306,874, 3,305,875 and 4,011,200, the disclosures of which are incorporated herein by reference.
The polyvinyl aromatic constituent is commonly a polystyrene, such as high-impact polystyrene. The term, "high-impact polystyrene" is used in the art to identify rubber modified polystyrene and is conventionally referred to by the acronym, HIPS. The rubber modifiers which are useful in this application are several in number, with polybutadiene, styrene-butadiene random and block copolymers, and ethylene-propylene diene terpolymers being exemplary. See U.S. Pat. Nos. 4,128,602, 4,128,603 and 4,128,604.
Numerous blends of polyphenylene ethers and polyvinyl aromatics are known. See, for example, U.S. Pat. Nos. 3,383,435, 3,887,646, 3,933,941, 3,994,856, 4,038,343, 4,128,602, 4,128,603 and 4,128,604, the disclosures of which are incorporated herein by reference.
In many applications, engineering thermoplastics are best provided with a flame retardant quality. For formulations based upon polyphenylene ether and polyvinyl aromatic blends, a particularly useful flame retardant for addition to the formulation is nuclear brominated polystyrene. In addition, flame retardant synergists, such as Sb.sub.2 O.sub.3, are often used in cooperation with such halogenated polyphenylene ethers. While the flame retardancy of such formulations is very high, e.g. a UL-94 rating of V-0, these formulations have a very low melt flow, e.g. a melt index of about 3.0 g/10 min. ASTM 1238. With this low of a melt flow, processing of these formulations is difficult at best and productivity may suffer as a result.
The melt flow of such formulations can be significantly improved by the use of plasticizer-type materials, such as pentaerythritol ester. Such plasticizers are not necessarily a panacea though, as many increase the flammability of the formulation and contribute to a lowering of the heat deflection temperature of articles made from the plasticizer containing formulation.
It is therefore an object of this invention to provide formulations which include a blend of polyphenylene ether and polyvinyl aromatic, a halogenated polyphenylene ether flame retardant, and, optionally, a flame retardant synergist, which formulations have both improved melt flow and high flame retardancy.